The present invention relates to micro-optical elements that rotate bi-directionally around one or two axes of rotation. More specifically, this invention relates to micro-opto-electro-mechanical (MOEM) devices that sense and electrostatically control the angular position of an optical element.
Improved and more robust micromachined beam steering mirrors with high angular position accuracy are desired for fiber optics communications networks, free-space laser communications systems, laser radars, and projection displays. Micromirror arrays can be used for cross-connect switches and add/drop multiplexers in all-optical networks. Mirrors controlled by electrostatic force feedback around two axes of rotation over large angular ranges can enhance the performance of compact scanning, tracking, pointing, imaging, image stabilization, laser marking, and laser micromachining systems.
The micromirrors of many prior-art optical switches have one or two stable positions in which to redirect an optical beam to a designated output. Optical switches with torsional mirrors can route an optical signal from an input fiber to any one of N output fibers in a 1-D array. A mirror mounted by thin-film, torsional flexure beams in a double gimbal arrangement can be positioned around two axes of rotation to route an optical signal to any fiber in a bundle of Nxc3x97M fibers in a 2-D array.
Mirrors suspended by torsional flexures and gimbal frames are angularly displaced by electrostatic torque developed by applying a voltage across the electrodes of a variable air-gap capacitor. Capacitor electrodes are formed on a surface of the mirror and cooperating electrodes are attached to stationary structure. Four pairs of cooperating capacitor electrodes are required to angularly position an optical element bi-directionally around two axes of rotation.
A disadvantage of controlling a micromirror by a variable air-gap capacitor is the narrow spacing between the capacitor electrodes limits the displacement of the movable electrode. This displacement is further restricted by the well-known xe2x80x9cpull-inxe2x80x9d instability that occurs at a critical voltage at which the movable electrode deflects by about ⅓ of the un-deflected capacitor gap. Electrode collapse arises due to the highly nonlinear force of attraction between the capacitor electrodes with applied voltage. This force varies as the inverse of the gap spacing squared while the elastic reaction torque of flexure means remains substantially linear over allowable angles of mirror tilt.
It is known that a control voltage superimposed on a larger fixed bias voltage improves the ability to control a torsional mirror over a small range of angles. It is also well know that differential capacitors can further improve force linearity as disclosed by Uchimaru, U.S. Pat. No. 5,740,150. However, the difficulties, limitations, and electronic complexity of obtaining a reasonably well behaved response for a two-axis, micromachined beam steering mirror over a practical angular range of tip and tilt were demonstrated analytically and experimentally by Toshiyoshi, et al., xe2x80x9cLinearization of Electrostatically Actuated Surface Micromachined 2-D Optical Scanner,xe2x80x9d J. Micro Electro Mech Syst. vol. 10, no. 2, 2001. This difficulty is compounded because the non-linear force-angle characteristic of a mirror driven by air-gap capacitor actuator is dependent upon both the angular position and vertical displacement of the mirror element.
P. F. Van Kessel, et. al.,xe2x80x9cMEMS-Based Projection Display,xe2x80x9d Proc. IEEE, vol. 86, August 1998, describe a digital micromirror device (DMD) comprising an array of thin-film, torsional mirror elements. The mirrors are rapidly switched between two stable states of deflection to spatially modulate light for image projection. Although the mirror elements are deflected to angles of about xc2x110xc2x0, the problem of a non-linear electrostatic transfer function is accommodated. When the DMD mirror is tilted away from its relaxed state, a leading edge of the mirror mechanically lands on a surface beyond the control electrodes to prevent total electrostatic collapse.
Another short coming of prior-art, micromirror arrays is that the mirror elements and support structure are generally micromachined from thin-films, e.g., polysilicon or metals. It is difficult to control film stresses, bending, and out-of-plane distortion of components constructed of these materials. Micromirrors suspended by double gimbals with two pairs of torsional flexure beams are complex devices to fabricate and are difficult to control because the non-linear response is also coupled to bending deformations.
The advantages of electrostatically controlled actuators with curved electrodes are well known, e.g., Legtenberg, et. al., xe2x80x9cElectrostatic Curved Electrode Actuators,xe2x80x9d Proc. IEEE Conf. on Micro Electro Mechanical Syst., Amsterdam, The Netherlands, January-Febuary, 1995. These actuators operate at substantially lower bias and control voltages than actuators with variable air-gap capacitors.
The variable capacitor of U.S. Pat. No. 6,151,967 with a contoured stationary electrode can be operated as an electrostatic actuator as disclosed in xe2x80x9cForce-Balanced Capacitive Transducer,xe2x80x9d U.S. patent application Ser. No. 09/866,351, May 25, 2001. This capacitor is referred to herein as variable area capacitor (VAC) since a substantial portion of a change in capacitance with a applied force is due to an increase in effective electrode area rather than a change in electrode spacing. The capacitance of a VAC increases as an area of fixed capacitive spacing increases between cooperating electrodes while the approach of a movable electrode with respect to a stationary electrode remains small.
An advantage of sensors and actuators with regions of fixed dielectric capacitance spacing between cooperating electrodes is the very large capacitance change and high values of quiescent capacitance typical of these transducers. This results in several orders of magnitude increased dynamic range. High quiescent capacitance avoids the noise limitations of small capacitors and associated detection electronics as well as the reduction of transducer sensitivity due to parasitic capacitance. Problems associated with the pickup of stray signals are reduced if one electrode of a variable capacitor, or the common electrode of a differential variable capacitor is grounded.
Accordingly, optical mirrors and switches of simple construction are desired that bi-directionally position an optical beam around two axes of rotation to high angular resolution and accuracy; operate at low bias and control voltages over practical angular ranges; and are micromachined from silicon or another a high strength material with stable mechanical properties.
The general object of the present invention is to provide an opto-electro-mechanical transducer, a method of construction, and a method to control the angular position of a rigid body without the performance limitations of prior-art transducers with variable air-gap capacitors. The rigid body can include a mirror, lens, grating, filter, holographic element, electrical component, or mechanical component. The embodiments of the present invention employ variable capacitors with regions of fixed capacitance spacing between cooperating electrodes to develop greater electrostatic forces and larger displacements at low operating voltages compared to prior-art capacitors actuators.
A specific objective is to provide micromirrors and optical switches having a substantially linear equilibrium force-angle response characteristic that can be controlled to high angular accuracy by closed-loop electrostatic force feedback over a range of tip and tilt angles.
Another objective is to provide a transducer with structural means that allows an optical element to bi-directionally rotate around two axes rotation without the complexity of a gimbal frame and position varying voltage compensation.
Still another objective is to provide a method to micromachine MOEMS with micro-optical elements from single-crystal silicon or another a high strength material with stable mechanical properties.
A further objective is to provide a method of control that allows an electrode of an electrostatic actuator to be electrically grounded and a cooperating electrode to simultaneously sense and control the angular position of an optical element. And alternately, provide a differential opto-electro-mechanical transducer with differential sense and control electrodes and a common ground electrode.